Deselected TileMap does not remember previously selected tile source

[KoBeWi]

Godot version

4.0 rc2

System information

Windows 10 x64

Issue description

Regression from #71770

When you have a TileMap with multiple tile sources and deselect the node, it resets back to 0. It's likely because the editor edits null and resets to initial state.

This wasn't happening before.

Steps to reproduce

1. Add TileMap
2. Add multiple tile sources
3. Select any source other than the first one
4. Click other node
5. Click TileMap again
6. Your source is no longer selected

Minimal reproduction project

N/A

[KoBeWi]

It's happening again.